As the concept of asynchronous transfer mode (ATM) gains wide acceptance, network designers are facing the issue known as "the last mile problem". There are cost effective ways to switch user information locally, i.e. LAN and voice switches, just as there are cost-effective ways to interconnect cities, i.e. fiber optics cables carrying synchronous digital hierarchy (SDH) traffic, which can serve as trunks for ATM switches. The "last mile" is the region between the user and the wide area network.
There are two classes of last mile access: dedicated links and shared media. Dedicated links include the copper twisted pairs (possibly with ADSL enhancement), some coax installations and point to point radios. Shared media include point to multipoint radio, coax, and some fiber optics solutions. Shared media has two main advantages: first, cost reduction at the head-end by serving many customers with a single unit; and second, the flexibility of dynamically allocating the shared capacity among the users based on instantaneous needs.
However, shared media poses several issues, such as how to maintain integrity if one user fails and floods the media with interfering signals and how to share the media fairly among the users. In particular, there is an issue of how to guarantee each user a quality of service (QoS) performance for each particular service, such as constant bit rate circuit emulation services (CBR-CES) and available bit rate (ABR).
These issues have been partially addressed by a family of techniques that include the following:
1. A base station (BS) controls the operation of the subscriber terminals (STs). PA1 2. Transmissions on the medium can be made from the BS to STs (downstream) or from ST to the BS (upstream) but not from ST to ST. PA1 3. The downstream transmission is a broadcast to all users, and the destination ST is specified by a media access control (MAC) address. PA1 4. The upstream transmission is moderated by the BS, and the BS specifies which ST will transmit at each given transmission opportunity (or "slot"). PA1 5. All user-generated traffic is transmitted based on grants from the BS, and no data is transmitted in a contention slot, such as slotted ALOHA. PA1 6. Contention slots or polling are used for bandwidth reservation. PA1 1. Requests and grants that include virtual circuit (VC) information, in which a VC represents a virtual path identifier/virtual channel identifier (VPI/VCI) of an ATM cell's flow. PA1 2. A periodical request-less per VC scheduler residing at the BS, called a "virtual framer". PA1 3. A request-based per VC scheduler residing at the BS, called a "virtual shaper".
The various techniques differ in the way they support multiple services in the upstream direction. For example, a customary technique is to define a time division multiplex (TDM) table such that each ST is allocated a few slots within the TDM table based on the user's traffic load. The TDM table includes contention slots for bandwidth requests and one-user slots for data transfer. However, the customary technique is too slow to respond to momentary bursts of traffic of specific users.
An alternative technique disclosed in application Ser. No. 08/708593 eliminates the TDM table at the ST and instead maintains the timing information at the BS for all users. In accordance with the alternative technique, the BS calculates the time intervals of CBR virtual circuits (VC) and queues a grant for each VC when its time has matured. Non-time-critical services such as ATM nrt-VBR send requests via contention slots or attached to any upstream cell MAC overhead. The requests carry a summary of the total buffer occupancy in the ST (excluding CBR), and the ST calculates an urgency figure for the buffer's status. The requests are prioritized by the BS. Once the BS decides which ST gets a grant, the grant is sent without specifying which VC within an ST can use the grant. Thus, the alternative technique provides non-directed grants.
Another example of scheduling ATM flow over a wireless network is discussed in the paper entitled "Guarantee Quality of Service Wireless Access ATM Networks" by C-S. Chang, K-C. Chen, M-Y You and J-F. Chang in IEEE J. Sel. Areas Com. Vol. 15. No. 1, January 1997, p. 106. The paper provides performance analysis of a wireless ATM network in which CBR transmission "tokens" are generated periodically and have higher priority than non-CBR traffic. Among the CBR tokens, the one selected for current transmission is the one with the highest static priority. This approach allows a bound to be calculated on the worst case delay of each VC. However, this approach lacks fairness, because a VC of similar quality objectives but lower priority will get a lower grade of service.